Method and apparatus for welding workpieces of high-temperature superalloys

ABSTRACT

A welding apparatus for welding workpieces of high-temperature superalloys is provided. The welding apparatus includes a heat source for producing a heat input zone on the workpiece surface, a supplying device for supplying welding filler into the heat input zone and a transporting device for producing a relative movement between the heat source and the supplying device on the one hand and the workpiece surface on the other hand. The welding apparatus further includes a control unit with a control program, which carries out the relative movement in such a way that the welding power and the diameter of the heat input zone are set such that the cooling rate during the solidifying of the material is at least 8000 Kelvins per second.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of German application No. 10 2009 049518.5 filed Oct. 15, 2009, which is incorporated by reference herein inits entirety.

FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for weldingworkpieces, in particular gas turbine workpieces, for example gasturbine blades.

BACKGROUND OF THE INVENTION

Moving blades of gas turbines are exposed to high temperatures andstrong mechanical loads during operation. Nickel-based superalloys,which can be strengthened by precipitation of a γ′ phase, are thereforeused with preference for such components. Nevertheless, cracks may occurover time in the moving blades and spread further as time progresses.Such cracks may be caused for instance by extreme mechanical loadingduring the operation of a gas turbine, but they may also already occurduring the production process. Since the production of turbine bladesand other workpieces of such superalloys is complex and cost-intensive,there are efforts to produce as little scrap as possible in productionand to ensure a long service life of the products produced.

Gas turbine blades that are in operation are routinely serviced andexchanged where necessary, if satisfactory functioning can no longer beensured with certainty because of operationally related loading. To makeit possible for exchanged turbine blades to be used further, whereverpossible they are refurbished. They can then be used once again in a gasturbine. Such refurbishment may, for example, involve the necessity forbuild-up welding in damaged regions, in order to restore the originalwall thickness.

Turbine blades which have already developed cracks during the productionprocess may, for example, be made fit for use by build-up welding, sothat scrap can be avoided in production.

However, at present, it is only with difficulty that the γ′-strengthenednickel-based superalloys can be welded with welding fillers of the sametype by means of conventional welding methods. The reason for this isthat micro-segregations, that is to say microscopic separations, of themolten material, must be avoided. Moreover, the welding process itselfcan lead to the generation of cracks in the welded region duringsubsequent heat treatments. These are caused by residual weldingstresses due to plastic deformations during the heat input when welding.

In order to circumvent the difficult weldability of the γ′-hardenednickel-based superalloys, welding is often performed with ductilewelding fillers, for instance with nickel-based alloys without γ′hardening. One such typical nickel-based alloy without γ′ hardening is,for example, IN 625. The ductility of the filler that is not γ′ hardenedallows the reduction of welding stresses due to plastic deformationsduring the first heat treatment after welding. However, the unhardenedalloys have lower high-temperature resistance (both low tensile strengthand low creep strength) in comparison with γ′-hardened nickel-basedsuperalloys. Therefore, welding methods without ductile fillers are usedwith preference. These methods may be divided into two classes, methodsin which overaging of the base material takes place to increase theductility by means of coarsening of the γ′ phase and methods in whichthe welding process is carried out with a preheated substrate. Carryingout the welding process on a preheated substrate avoids the residualwelding stresses by means of recovery during the welding process.

A welding process with prior overaging is described, for example, inU.S. Pat. No. 6,120,624, a welding process carried out on a preheatedworkpiece is described, for example, in U.S. Pat. No. 5,319,179.

However, the two mentioned welding methods without ductile weldingfillers likewise have disadvantages. For example, in the case ofoveraging carried out before the welding process, a corresponding heattreatment of the γ′-hardenable nickel-based superalloys is carried outbefore the welding, in order to bring about the overaging of the γ′phase. The ductility of the base material is thereby increasedsignificantly. This increase in the ductility makes it possible to weldthe material at room temperature. Moreover, it can be cold-straightened.Furthermore, such a heat treatment makes it possible for nickel-basedsuperalloys such as, for example, Rene 41 or Haynes 282 to be used as awelding filler. Although these form γ′ phases in the microstructure,they do so only with a significantly smaller proportion of the volumethan the typical γ′-containing nickel-based superalloys that are usednowadays for gas turbine hot-gas components, such as gas turbine blades(for example IN 738 LC, IN 939, Rene 80, IN 6203 DS, PWA 1483 SX, Alloy247, etc.). Therefore, even when overaging is performed before thewelding process, no full structural weldings take place.

If a preheating of the turbine blade is performed, the temperaturedifference and the resultant stress gradient between the weld point andthe rest of the turbine blade is reduced, whereby the formation ofwelding cracks in components of nickel-based superalloys can be avoided.Such methods in which preheating of the turbine blade to temperaturesbetween 900° C. and 1000° C. is performed by means of induction coilsmust, however, be carried out under shielding gas, which makes thewelding process complicated and expensive. Moreover, as a result of lackof accessibility to the workpiece located in a shielding gas enclosure,this method cannot be carried out on all regions of the workpiece.

SUMMARY OF THE INVENTION

There is therefore a need for an alternative welding method for build-upwelding that is suitable in particular for γ′-hardened nickel-basedsuperalloys and does not have the aforementioned disadvantages, or onlyto a reduced extent. It is a further object of the invention to providea welding apparatus suitable for carrying out the method according tothe invention.

The first object is achieved by a method for build-up welding, thesecond object is achieved by a welding apparatus as claimed in theindependent claims. The dependent claims contain advantageousrefinements of the invention and can be advantageously combined with oneanother as desired.

In the method according to the invention for welding workpieces ofhigh-temperature superalloys, welding filler is applied to the workpiecesurface by means of a heat input zone and a supply zone for supplyingthe welding filler into the heat input zone. The heat input zone and thesupply zone are moved over the workpiece surface during the welding. Themovement may take place along a welding direction, for example on alinear path or on a path oscillating around the welding direction. Inthe method according to the invention, the welding parameters are chosensuch that the cooling rate during the solidifying of the material is atleast 8000 K/s.

The main parameters available for setting the cooling rate of at least8000 K/s during the solidifying of the material are the methodparameters with respect to the welding power and the diameter of theheat input zone, for example in the form of a laser power and a diameterof the laser beam, the feed (the speed of the process) and possibly thestream of welding filler supplied. Depending on the type of laser sourceused, the required cooling rate for the material to be welded can be setby suitable adaptation of these parameters. The speed of the processhere may be at least 250 mm/min, particularly more than 500 mm/min. Forexample, with a speed of the process of more than 500 mm/min, the methodparameters with respect to the welding power and the diameter of theheat input zone can be set such that the cooling rate during thesolidifying of the material is at least 8000 K/s.

The high cooling rate and high solidifying rate have the effect ofincreasing the distribution coefficient to such an extent that microsegregations, i.e. microscopic separations, of the molten material arelargely avoided. The molten material in the weld metal solidifiesdendritically, that is to say in a tree-like structure, the directionsof growth of the dendrites varying along the welding trace, since theorientation of the possible directions of growth of the dendrites inrelation to the temperature gradient on the solidification front varies.That direction of growth with the smallest inclination in relation tothe temperature gradient or with the smallest rate of growth prevails.In addition, seeds form ahead of the solidification front, which catchesup with them during the solidification. These seeds initiate directionsof dendrite growth that are randomly distributed.

The method according to the invention is suitable, for example, forwelding workpieces of a γ′-containing nickel-based superalloy by meansof a welding filler which is a γ′-forming nickel-based superalloymaterial. A high strength can be achieved in the weld metal as a resultof the use of filler of the same type and acceptable welding quality canbe achieved, i.e. a very small number of cracks and a very low averagecrack length. As a result of the possibility of carrying out the weldingprocess at room temperature with a shielding gas atmosphere locallyaround the melting bath, the welding method according to the inventionbecomes highly cost-effective.

The method may be designed in particular as a build-up welding method,in which the welding filler is applied layer by layer. As this happens,the welding directions of successive layers may be turned with respectto one another, particularly by 90°. Turning the welding direction ofdifferent layers allows attachment defects between the layers to beavoided, in particular whenever the heat input zone and the supply zoneare also moved over the workpiece surface along the welding direction ona path that oscillates around the welding direction.

The irregularly distributed dendrite orientation is mainly in the upperhalf of the welding trace. Therefore, in the method according to theinvention, a previously applied layer is remelted in less than half itslayer thickness. In this case, the crystal structure of the remeltedregions is taken over during solidification. The small remelting depthensures that the solidification front comes down on a region withirregularly distributed dendrite orientations. In the case ofmulti-layer welding, this has the result that a polycrystal with grainsof a very small average diameter is generated. Grain boundariesgenerally represent a weakness with respect to crack formation undertransient stresses during the welding process or subsequent heattreatment. The low expansion of a grain boundary in the plane and itsirregular orientation in the weld metal welded by the method accordingto the invention have the effect that the weld metal is less sensitiveto crack formation, so that the welding process can be carried out atroom temperature.

The method according to the invention can be used both in the case ofpolycrystalline substrates and in the case of directionally solidifiedor monocrystalline substrates. In all the cases mentioned, aγ′-containing nickel-based superalloy may be used as the welding filler.

In the course of the welding method according to the invention, theapplication of the welding filler may be followed by a heat treatment.With a heat treatment adapted to the weld metal, the desired γ′morphology can be set. This serves for further improving the strength ofthe weld metal.

A welding apparatus according to the invention for weldinghigh-temperature superalloys that is suitable for carrying out themethod according to the invention comprises a heat source for producinga heat input zone on the workpiece surface, a supplying device forsupplying welding fillers into the heat input zone and a transportingdevice for producing a relative movement between the heat source and thesupplying device on the one hand and the workpiece surface on the otherhand. The transporting device is advantageously connected to the heatsource and the supplying device for the welding filler, in order to movethe heat source and the supplying device to bring about the relativemovement. This is generally less problematic than moving the workpiece.In the welding apparatus according to the invention, a laser may be usedin particular as the heat source. The welding apparatus according to theinvention also comprises a control unit with a control program, whichsets the welding parameters such that the cooling rate during thesolidifying of the material is at least 8000 Kelvins per second. Inparticular, the control unit can set the welding parameters with respectto the welding power and the diameter of the heat input zone such thatthe cooling rate during the solidifying of the material is at least 8000Kelvins per second. The welding can be carried out here with a speed ofthe process of at least 250 mm per minute, particularly with a speed ofthe process of more than 500 mm per minute.

The relative movement may be controlled in particular in such a way thatthe heat input zone and the supply zone are moved over the workpiecesurface along a welding direction on a path oscillating around thewelding direction. Furthermore, the control unit may carry out therelative movement with or without oscillation in such a way that thewelding directions for successive layers are turned with respect to oneanother, for example by 90°.

The welding apparatus according to the invention makes it possible tocarry out the welding method according to the invention by using acontrol program which contains the welding parameters for the weldingprocess described in conjunction with the method, for instance the pathof the relative movement between the heat source and the supplyingdevice on the one hand and the workpiece on the other hand, the speed ofthe process, the laser power, the beam diameter, etc. The methodparameters and mechanisms described in conjunction with the method helpto suppress the formation of cracks, such as solidification cracks orremelting cracks, in the base material and the molten material. This isalso particularly the case whenever both the base material and thewelding filler are γ′-forming nickel-based superalloys. This results ina quality of the welding that can be achieved with the method accordingto the invention and the welding apparatus according to the inventionthat is acceptable for structural welding, for instance for the purposeof repairing or joining in a highly stressed region of a turbine bladeor some other workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, properties and advantages of the present inventionemerge from the following description of exemplary embodiments withreference to the accompanying figures.

FIG. 1 shows by way of example a gas turbine in a longitudinal partialsection.

FIG. 2 shows a turbine blade in a perspective view.

FIG. 3 shows a gas turbine combustion chamber in a partially sectionedperspective representation.

FIG. 4 shows the welding apparatus according to the invention in aschematic representation.

FIG. 5 shows the welding path for a first layer of welding filler.

FIG. 6 shows the welding path for a second layer of welding filler.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows by way of example a gas turbine 100 in a longitudinalpartial section.

The gas turbine 100 has in the interior a rotor 103 with a shaft 101,which is rotatably mounted about an axis of rotation 102 and is alsoreferred to as a turbine runner.

Following one another along the rotor 103 are an intake housing 104, acompressor 105, a combustion chamber 110, for example of a toroidalform, in particular an annular combustion chamber with a number ofcoaxially arranged burners 107, a turbine 108 and the exhaust housing109.

The annular combustion chamber 110 communicates with a hot gas duct 111,for example of an annular form. There, the turbine 108 is formed forexample by four successive turbine stages 112.

Each turbine stage 112 is formed for example by two blade rings. As seenin the direction of flow of a working medium 113, a row of stationaryblades 115 is followed in the hot gas duct 111 by a row 125 formed bymoving blades 120.

The stationary blades 130 are in this case fastened to an inner housing138 of a stator 143, whereas the moving blades 120 of a row 125 areattached to the rotor 103, for example by means of a turbine disk 133.

Coupled to the rotor 103 is a generator or a machine (not represented).

During the operation of the gas turbine 100, air 135 is sucked in by thecompressor 105 through the intake housing 104 and compressed. Thecompressed air provided at the end of the compressor 105 on the turbineside is passed to the burners 107 and mixed there with a fuel. Themixture is then burned in the combustion chamber 110 to form the workingmedium 113. From there, the working medium 113 flows along the hot gasduct 111 past the stationary blades 130 and the moving blades 120. Atthe moving blades 120, the working medium 113 expands, transferringmomentum, so that the moving blades 120 drive the rotor 103 and thelatter drives the machine coupled to it.

The components that are exposed to the hot working medium 113 aresubjected to thermal loads during the operation of the gas turbine 100.The stationary blades 130 and moving blades 120 of the first turbinestage 112, as seen in the direction of flow of the working medium 113,are thermally loaded the most, along with the heat shielding elementslining the annular combustion chamber 110.

In order to withstand the temperatures prevailing there, these may becooled by means of a coolant.

Similarly, substrates of the components may have a directionalstructure, i.e. they are monocrystalline (SX structure) or only havelongitudinally directed grains (DS structure).

Iron-, nickel- or cobalt-based superalloys are used for example as thematerial for the components, in particular for the turbine blade 120,130 and components of the combustion chamber 110.

Such superalloys are known, for example, from EP 1 204 776 B1, EP 1 306454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949.

Similarly, the blades 120, 130 may have coatings against corrosion(MCrAlX; M is at least one element of the group comprising iron (Fe),cobalt (Co) and nickel (Ni), X is an active element and representsyttrium (Y) and/or silicon, scandium (Sc) and/or at least one element ofthe rare earths, or hafnium). Such alloys are known from EP 0 486 489B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1.

A thermal barrier coating, which consists for example of ZrO₂,Y₂O₃—ZrO₂, i.e. is unstabilized, partially stabilized or completelystabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide,may also be present on the MCrAlX.

Columnar grains are produced in the thermal barrier coating by suitablecoating methods, such as for example electron-beam physical vapordeposition (EB-PVD).

The stationary blade 130 has a stationary blade root (not representedhere), facing the inner housing 138 of the turbine 108, and a stationaryblade head, at the opposite end from the stationary blade root. Thestationary blade head faces the rotor 103 and is fixed to a fasteningring 140 of the stator 143.

FIG. 2 shows in a perspective view a moving blade 120 or stationaryblade 130 of a turbomachine, which extends along a longitudinal axis121.

The turbomachine may be a gas turbine of an aircraft or of a power plantfor generating electricity, a steam turbine or a compressor.

The blade 120, 130 has, following one after the other along thelongitudinal axis 121, a fastening region 400, an adjoining bladeplatform 403 and also a blade airfoil 406 and a blade tip 415.

As a stationary blade 130, the blade 130 may have a further platform atits blade tip 415 (not represented).

In the fastening region 400 there is formed a blade root 183, whichserves for the fastening of the moving blades 120, 130 to a shaft or adisk (not represented).

The blade root 183 is designed for example as a hammer head. Otherdesigns as a firtree or dovetail root are possible.

The blade 120, 130 has for a medium which flows past the blade airfoil406 a leading edge 409 and a trailing edge 412.

In the case of conventional blades 120, 130, solid metallic materials,in particular superalloys, are used for example in all the regions 400,403, 406 of the blade 120, 130.

Such superalloys are known, for example, from EP 1 204 776 B1, EP 1 306454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949.

The blade 120, 130 may in this case be produced by a casting method,also by means of directional solidification, by a forging method, by amilling method or combinations of these.

Workpieces with a monocrystalline structure or structures are used ascomponents for machines which are exposed to high mechanical, thermaland/or chemical loads during operation.

The production of monocrystalline workpieces of this type takes placefor example by directional solidification from the melt. This involvescasting methods in which the liquid metallic alloy solidifies to formthe monocrystalline structure, i.e. to form the monocrystallineworkpiece, or in a directional manner.

Dendritic crystals are thereby oriented along the thermal flow and formeither a columnar grain structure (i.e. grains which extend over theentire length of the workpiece and are commonly referred to here asdirectionally solidified) or a monocrystalline structure, i.e. theentire workpiece comprises a single crystal. In these methods, thetransition to globulitic (polycrystalline) solidification must beavoided, since undirected growth necessarily causes the formation oftransversal and longitudinal grain boundaries, which nullify the goodproperties of the directionally solidified or monocrystalline component.

While reference is being made generally to solidified structures, thisis intended to mean both monocrystals, which have no grain boundaries orat most small-angle grain boundaries, and columnar crystal structures,which indeed have grain boundaries extending in the longitudinaldirection but no transversal grain boundaries. These second-mentionedcrystalline structures are also referred to as directionally solidifiedstructures.

Such methods are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1.

Similarly, the blades 120, 130 may have coatings against corrosion oroxidation, for example (MCrAlX; M is at least one element of the groupcomprising iron (Fe), cobalt (Co) and nickel (Ni), X is an activeelement and represents yttrium (Y) and/or silicon and/or at least oneelement of the rare earths, or hafnium (Hf)). Such alloys are known fromEP 0 486 489 B1, EP 0 786 017 B1, EP 0 412397 B1 or EP 1 306454 A1.

The density is preferably 95% of the theoretical density.

A protective aluminum oxide layer (TGO=thermal grown oxide layer) formson the MCrAlX layer (as an intermediate layer or as the outermostlayer).

The composition of the layer preferably comprisesCo-30Ni-28Cr-8Al-0.6Y-0.7Si or Co-28Ni-24Cr-10Al-0.6Y. Apart from thesecobalt-based protective coatings, nickel-based protective coatings arealso preferably used, such as Ni-10Cr-12Al-0.6Y-3Re orNi-12Co-21Cr-11Al-0.4Y-2Re or Ni-25Co-17Cr-10Al-0.4Y-1.5Re.

A thermal barrier coating which is preferably the outermost layer andconsists for example of ZrO₂, Y₂O₃—ZrO₂, i.e. is unstabilized, partiallystabilized or completely stabilized by yttrium oxide and/or calciumoxide and/or magnesium oxide, may also be present on the MCrAlX.

The thermal barrier coating covers the entire MCrAlX layer.

Columnar grains are produced in the thermal barrier coating by suitablecoating methods, such as for example electron-beam physical vapordeposition (EB-PVD).

Other coating methods are conceivable, for example atmospheric plasmaspraying (APS), LPPS, VPS or CVD. The thermal barrier coating may havegrains which are porous, are provided with microcracks or are providedwith macrocracks for better thermal shock resistance. The thermalbarrier coating is therefore preferably more porous than the MCrAlXlayer.

Refurbishment means that components 120, 130 may have to be freed ofprotective layers after use (for example by sandblasting). This isfollowed by removal of the corrosion and/or oxidation layers orproducts. If need be, cracks in the component 120, 130 are then alsorepaired. This is followed by recoating of the components 120, 130 andrenewed use of the components 120, 130.

The blade 120, 130 may be hollow or be of a solid form. If the blade120, 130 is to be cooled, it is hollow and may also have film coolingholes 418 (indicated by dashed lines).

FIG. 3 shows a combustion chamber 110 of a gas turbine. The combustionchamber 110 is designed for example as what is known as an annularcombustion chamber, in which a multiplicity of burners 107, whichproduce flames 156 and are arranged in the circumferential directionaround an axis of rotation 102, open out into a common combustionchamber space 154. For this purpose, the combustion chamber 110 isdesigned as a whole as an annular structure, which is positioned aroundthe axis of rotation 102.

To achieve a comparatively high efficiency, the combustion chamber 110is designed for a comparatively high temperature of the working medium Mof approximately 1000° C. to 1600° C. To permit a comparatively longoperating time even with these operating parameters that are unfavorablefor the materials, the combustion chamber wall 153 is provided on itsside facing the working medium M with an inner lining Rained by heatshielding elements 155.

Each heat shielding element 155 of an alloy is provided on the workingmedium side with a particularly heat-resistant protective layer (MCrAlXlayer and/or ceramic coating) or is produced from material that isresistant to high temperature (solid ceramic bricks).

These protective layers may be similar to the turbine blades, meaningfor example MCrAlX: M is at least one element of the group comprisingiron (Fe), cobalt (Co) and nickel (Ni), X is an active element andrepresents yttrium (Y) and/or silicon and/or at least one element of therare earths, or hafnium (Hf). Such alloys are known from EP 0 486 489B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1.

A thermal barrier coating which is for example a ceramic thermal barriercoating and consists for example of ZrO₂, Y₂O₃—ZrO₂, i.e. isunstabilized, partially stabilized or completely stabilized by yttriumoxide and/or calcium oxide and/or magnesium oxide, may also be presenton the MCrAlX.

Columnar grains are produced in the thermal barrier coating by suitablecoating methods, such as for example electron-beam physical vapordeposition (EB-PVD).

Other coating methods are conceivable, for example atmospheric plasmaspraying (APS), LPPS, VPS or CVD. The thermal barrier coating may havegrains which are porous, are provided with microcracks or are providedwith macrocracks for better thermal shock resistance.

Refurbishment means that heat shielding elements 155 may have to befreed of protective layers after use (for example by sandblasting). Thisis followed by removal of the corrosion and/or oxidation layers orproducts. If need be, cracks in the heat shielding element 155 are thenalso repaired. This is followed by recoating of the heat shieldingelements 155 and renewed use of the heat shielding elements 155.

On account of the high temperatures in the interior of the combustionchamber 110, a cooling system may also be provided for the heatshielding elements 155 or for their holding elements. The heat shieldingelements 155 are for example hollow and, if need be, also have coolingholes (not represented) opening out into the combustion chamber space154.

FIG. 4 shows a welding apparatus 1 according to the invention in ahighly schematized representation.

This apparatus comprises a laser 3 and a powder supplying device 5, withwhich a powdered welding filler can be applied to the region to bewelded of a workpiece 9. By means of the laser radiation, a heat inputzone 11 is formed on the workpiece surface, into which zone the powder13 is introduced by the powder supplying device 5. The laser ispreferably a 300 W laser, particularly an Nd-YAG laser, mostparticularly with λ=1.06 μm. The laser power is between 100 W and 300 Wand preferably between 100 W and 200 W, most particularly between 100 Wand 150 W. The welding material is thus advantageously melted well andalso fused well to the underlying surface, in order that a sealed weldpoint is obtained.

The laser 3 and the powder supplying device 5 are arranged on a scanningdevice 15, which makes it possible for the laser 3 and the powdersupplying device 5 to be displaced in two dimensions along the componentsurface (x and y directions in FIG. 4) with the region 7 to be welded.The speed of the process is at least 250 mm/min, particularly 400mm/min-600 mm/min, most particularly 500 mm/min. An advantageous heatinput into the welding material and the substrate is thus possible.

Furthermore, the scanning device 15 of the present exemplary embodimentmakes it possible for the laser 3 and the powder supplying device 5 tobe displaced perpendicularly in relation to the component surface (zdirection in FIG. 4). With the aid of the scanning device 15, the heatinput zone and the impingement zone of the powder can consequently bedisplaced along a predetermined path. A robot acne may be used forexample as the scanning device. The diameter of the laser beam isparticularly 500 μm to 700 μm, most particularly 600 μm. Consequently,the welding material supplied can be advantageously heated.

The controlling of the movement instigated by the scanning device 15 isperformed by a control unit 17, which also controls the other parametersof the welding process. As a difference from the present exemplaryembodiment, the controlling of the other parameters of the weldingprocess may, however, also be performed by an additional control, thatis to say separately from the control of the movement sequence.Furthermore, as a difference from the exemplary embodiment represented,a movable component holder may also be used instead of the scanningdevice 15 for moving the laser 3 and the powder supplying device 5. Allthat is important for the invention is the relative movement between thelaser 3 and the powder supplying device 5 on the one hand and theworkpiece 9 on the other hand.

The method according to the invention for the build-up welding of aworkpiece surface may be used for material application, in particularfor multi-layer material application, to the region 7 to be welded of acomponent 9. In this case, the component 9 needs neither to be preheatednor to be overaged by means of a heat treatment.

The method is described below on the basis of build-up welding on thesurface 10 of a turbine blade 9 as a workpiece. The turbine blade of thepresent exemplary embodiment consists of a γ′-strengthened nickel-basedsuperalloy, for example of IN 738 LC, IN 939, Rene 80, IN 6203 DS, PWA1483 SX, Alloy 247, etc. The region 7 to be welded in the surface 10 ofthe turbine blade 9 is welded by building up layer after layer, the heatinput zone being moved together with the impingement region for thepowder 13 along a welding direction over the region 7 to be welded ofthe turbine blade 9. The powder 13 is in the present case a powder of aγ′-containing nickel-based superalloy, for example of IN 78 LC, IN 939,Rene 80, INT 6203 DS, PWA 1483, Alloy 247, etc.

The path P1 followed by the heat input zone 11 and the impingementregion of the powder 13 during the build-up welding of the first layeron the region 7 to be welded is schematically represented in FIG. 5. Thefigure shows the turbine blade 9 with the region 7 to be welded and thewelding direction S1 during the build-up welding of the first layer 19.The heat input zone 11, which at the same time represents theimpingement region for the powder 13, is however not displaced linearlyalong the welding direction S1 but instead it simultaneously oscillatesduring the displacement along the welding direction in a directionperpendicular to the welding direction. As a result, the heat input zone11 and the impingement region of the powder 13 follow a meandering pathP1 over the region 7 to be welded.

For the build-up welding of the second layer 21 (FIG. 4), the laser 3and the powder supplying device 5 are displaced a little along the zdirection of the scanning device 15. Furthermore, in the presentexemplary embodiment, the welding direction S2 is turned by 90° withrespect to the welding direction S1 for the first layer. The path P2 ofthe heat input zone 11 and of the impingement region for the powder 13during the build-up welding of the second layer 21 is represented inFIG. 6. Also during the build-up welding of the second layer 21, theheat input zone 11 oscillates together with the impingement region ofthe powder 13 in a direction perpendicular to the welding direction S2.Therefore, a meandering path P2 of the heat input zone 11 and theimpingement region for the powder 13 over the region 7 to be welded isobtained overall.

The paths described in conjunction with the exemplary embodimentrepresent only one of various possible variants. In principle, there areseveral possibilities for carrying out the welding: 1. unidirectional or2. bidirectional (for example meandering) build-up welding. In the caseof each of these variants, the traces (paths) of the 2nd layer may bewelded in a parallel offset or perpendicular manner in relation to thetraces (paths) of the first layer. All of these variants may be usedwithin the scope of the method according to the invention.

When moving the laser and the powder supplying unit, the oscillation maybe chosen such that the entire region 7 to be welded is passed over witha single path along the welding direction, as represented in FIG. 5, orsuch that only part of the region 7 to be welded is passed over and anumber of paths P2 running next to one another in the welding directionS2 are traveled for the build-up welding of the region as a whole, asrepresented in FIG. 6.

The moving of the heat input zone 11 and the impingement region of thepowder 13 along the path P1 or P2 is performed in the present exemplaryembodiment with a speed of the process of at least 500 mm/min. The laserpower, the beam diameter and the powder stream are in this case chosensuch that the cooling rate of the region passed over during thesolidification is greater than 8000 K/s. During the building up of thesecond layer 21, the process parameters with respect to laser power andbeam diameter are also chosen such that the remelting depth to which thefirst layer 19 is remelted is less than 50% of the trace height of thefirst layer 19. The remelting depth is indicated in FIG. 4 by dashedlines. In principle, speeds of the process other than that specified inthe present example are also possible, the other parameters, laserpower, beam diameter and powder stream, then having to be adaptedcorrespondingly.

The high cooling rate and high solidifying rate have the effect ofincreasing the distribution coefficient to such an extent that microsegregations are largely avoided. The molten material brought about bythe heat input zone 11 solidifies dendritically, the crystal structurebeing taken over from the crystal structure present in the remeltedregion. In this case, the directions of growth of the dendrites varyalong a path P1, P2. The reason for this is that the orientation of thepossible directions of growth of the dendrites in relation to thetemperature gradient varies, the direction of growth with the smallestinclination in relation to the temperature gradient or with the smallestrate of growth prevailing. Furthermore, seeds which form ahead of thesolidification front and are caught up by the solidification frontduring the solidification initiate directions of dendrite growth thatare randomly distributed. These irregularly distributed dendriteorientations are predominantly in the upper half of a layer 19. Thesmall remelting depth ensures that the solidification front comes downon a region with irregularly distributed dendrite orientations, which inthe case of multi-layer welding has the result that a polycrystal withgrains of a very small average diameter is generated. As a result, thewelded region of the turbine blade 9 is insensitive to crack formation.

Once the application of the required number of layers 19, 21 has takenplace, the turbine blade 9 can be subjected to a heat treatment whichproduces the desired γ′ morphology. This serves for further improvingthe strength of the welded region of the turbine blade 9.

With the method according to the invention, build-up welding can beperformed at room temperature and without prior overaging of thecomponent to be welded, the occurrence of solidification cracks andremelting cracks being suppressed. This results in a quality of thewelding that is acceptable for structural welding, in particular forhighly stressed regions of gas turbine blades, but also of othercomponents. At the same time, there is only very little influencing ofthe base material, since there is only very little heat input into thesubstrate as a result of the small heat-affected zone (no preheatingtakes place) and the suppression of remelting cracks in theheat-affected zone.

The invention claimed is:
 1. A method for welding a workpiece ofhigh-temperature superalloy, comprising: applying a welding filler to aworkpiece surface by a heat input zone; supplying the welding fillerinto the heat input zone by a supply zone; moving the heat input zoneand the supply zone in relation to the workpiece surface; and choosing awelding parameter so that a cooling rate during solidifying is at least8000 Kelvins per second, wherein the workpiece comprises a γ′-containingnickel-based superalloy and the welding filler is a γ′-formingnickel-based superalloy material, wherein the welding parameter isselected from the group consisting of: a welding power, speed of awelding process, and a diameter of the heat input zone, wherein thespeed of the welding process is at least 250 mm/min, wherein a weldingbeam comprising a laser is applied on the workpiece surface to producethe heat input zone, wherein a laser power is selected in a range of 100W to 300 W, and wherein a diameter of a laser beam is 500 μm to 800 μm.2. The method as claimed in claim 1, wherein a weld seam is produced byapplying the welding filler layer by layer, and wherein a previouslyapplied layer is remelted in less than half of its layer thickness. 3.The method as claimed in claim 2, wherein the heat input zone and thesupply zone are moved in relation to the workpiece surface along awelding direction for each layer, and wherein welding directions ofsuccessive layers are turned with respect to one another.
 4. The methodas claimed in claim 1, wherein the heat input zone and the supply zoneare moved in relation to the workpiece surface along a welding directionon a path oscillating around the welding direction.
 5. The method asclaimed in claim 1, wherein a heat treatment is applied after applyingthe welding filler.
 6. The method as claimed in claim 1, wherein thelaser comprises an Nd-YAG laser with λ=1.06 μm.
 7. The method as claimedin claim 1, wherein a polycrystalline weld seam is achieved.
 8. Awelding apparatus for welding a workpiece of high-temperaturesuperalloy, comprising: a heat source for producing a heat input zone ona workpiece surface; a supplying device for supplying a welding fillerinto the heat input zone; a transporting device for moving the heatsource and the supplying device in relation to the workpiece surface;and a control unit for setting a welding parameter so that a coolingrate during solidifying is at least 8000 Kelvins per second, wherein thecontrol unit sets the welding parameter that is selected from the groupconsisting of: a welding power, speed of a welding process, and adiameter of the heat input zone, and wherein the workpiece comprises aγ′-containing nickel-based superalloy and the welding filler is aγ′-forming nickel-based superalloy material.
 9. The welding apparatus asclaimed in claim 8, wherein the control unit moves the heat source andthe supplying device in relation to the workpiece surface with a speedof at least 250 mm per minute.
 10. The welding apparatus as claimed inclaim 8, wherein the control unit moves the heat input zone and thesupply zone in relation to the workpiece surface along a weldingdirection on a path oscillating around the welding direction.
 11. Thewelding apparatus as claimed in claim 8, wherein a weld seam is producedby applying the welding filler layer by layer along a welding direction,and wherein the control unit turns welding directions of successivelayers with respect to one another.
 12. The welding apparatus as claimedin claim 8, wherein the heat source comprises a 300 W laser.
 13. Thewelding apparatus as claimed in claim 12, wherein the laser comprises anNd-YAG laser with λ=1.06 μm.